1. Field of the Invention
This invention relates to integrated circuits (ICs) and more specifically to a prognostic cell capable of predicting impending failure of a useful circuit on the IC.
2. Description of the Related Art
All manufactured products have non-zero failure probability, that is, there is at every instance of use a certain probability that the product will fail. A particular product line's failure rate is the number of such products that is expected to fail per unit time. The failure rate is thus equivalent with the failure probability.
The failure probability during the life of a manufactured product typically follows a bathtub curve 10 as shown in FIG. 1. The bathtub curve contains three distinct regions: (i) an initial region 12 where the failure probability is high, called the burn-in or infant mortality region, (ii) a useful life region 14 where the failure rate is at its minimum, and (iii) an end-of-life region 16 where the product's failure rate starts to increase due to component wear. The bathtub curve can be measured for a particular product line when that product is used under well-defined conditions. This allows a manufacturer to specify (guarantee) a product's lifetime. A product's useful lifetime is typically defined as ending when the failure rate (or equivalently, failure probability) starts to increase due to wear. Wear is a function of time, intensity of use and environment. For microelectronic devices, the environment includes variables such as temperature, operating voltage, operating current, incident radiation, humidity, presence of corrosives etc.
The actual lifetime of a product used in the field may be very different than the lifetime measured under controlled and specified conditions. Part of that difference is due to variability of individual instances of a product (individual parts) within a particular product line. However, a large determining factor for actual part lifetime is determined by the place and method in which that part is used. This means that the bathtub curve for a particular part (which is the probability that that particular instance of a product will fail as a function of time) can be shifted substantially relative to the baseline.
For electronics applications where system reliability is important, system designers often assume worst-case conditions for reliability calculations. Integrated circuits can then be selected that meet the worst-case requirements. However, worst-case conditions are typically poorly defined and are usually not continuously present. Therefore, a worst-case design approach often results in over-specification of reliability requirements. Sometimes no integrated circuits are available that meet the system reliability requirements. In that case the system designer must build in redundancy to meet the reliability goal.
Latent weakness in the product due to non-idealities of the manufacturing process or due to mishandling of the product prior to or during use can also result in early failure. In effect, latent weakness has shifted the part along the time axis of the bathtub curve.
It would be particularly useful if a product could be equipped with a monitor that can determine if it has entered, or is about to enter, the wearout region of its life. Such a monitor is called a prognostic cell because it is capable of predicting impending failure. Preliminary work on development of prognostic cells was performed by Rome Laboratory (Air Force Materiel Command) as described in V.C. Tyree, “Self stressing test structure cells”, Rome Laboratory, Air Force Materiel Command, New York February 1995. These preliminary cells (RADC TDDB cells) provided no metric or methodology for relating the amount of excess stress applied to stress the prognostic cell to the remaining useful life of the host circuit. Furthermore, these cells provided no technique for addressing the wide distributions associated with IC failures.